Flexure supports are used to suspend a micromechanical device from a substrate. In accelerometer and gyro applications the micromechanical device contains a proof mass. In pendulous accelerometers the proof mass is suspended from a substrate by flexures which extend beyond the longitudinal edges of the proof mass to anchors mounted on the substrate. A strain relief beam is formed in the flexure support region and this serves to minimize the fabrication stresses that are transferred to the proof mass from the substrate. In order to make the device both sensitive at the milli-g level and manufacturable with a single silicon thickness the flexure is narrow in width and has a high aspect ratio.
Although the strain relief beam used gives an acceptable level of stress on the proof mass, after fabrication the structure is not thermally stable: the anchor regions are spaced relatively far apart and there is a thermal stress imparted by the glass substrate to the flexures. The strain relief beam mitigates the situation from the standpoint of structural fracture but the concern is that small imperfections in the structure result in a rotational stress which tilts the proof mass under thermal load. Any tilt of this nature is differential and will show up as a bias drift. Such a tilt will likely be very small. However, the accelerometer is extremely sensitive.
Because of the inherent stability of silicon a millimeter sized device can routinely sense accelerations in the milli-g level. With the typical device dimensions used this means sensing an acceleration induced tilt of about 1 Angstrom per milli-g. In other words the device bias is extraordinarily sensitive to thermally induced rotation of the proof mass.
An attempt was made recently to support a micromechanical device or proof mass on a single anchor. In theory a single point mount will decouple all stress between the glass substrate and the silicon device. The single point mount is acceptable in a device which is very stiff and measures acceleration in the 10 to 100 g level, for example. However, when the flexures must be made weaker to sense acceleration at the milli-g level a problem develops with rotational stiffness. If a single thickness of silicon is used the flexure has high aspect ratio and a narrow width; the device becomes very weak in rotation, and has a low yield in fabrication due to breakage.